The fact that childhood obesity rates and air pollution have generally plateaued could have impacted the number of cases, reports the 2001-2013 study. Overall, rates for kids 17-years-old and younger increased by a small amount, but then leveled and declined by the time the study ended, at which time 8.3% of children were affected. The numbers deviated among some ages, races, and regions.

Because of increased awareness and diagnosis, kids’ asthma rates doubled from 1980 to 1995. After that, increases slowed down by 2010. Demographics that experienced declines in recent years were those under five-years-old, Mexican children, kids in the Midwest, and children from families that were above the poverty level.

White youth and kids living in the West and Northeast plateaued, but those aged 10 to 17, children from families at the poverty level, and young people in the South showed increased rates. Among black youth, rates increased but then reached a plateau.

For the study, parents of over 150,000 kids were surveyed through an annual in-person government health poll in which they were asked if their sons and daughters had been diagnosed with asthma.

According to information not included in the study, the 2014 rates rose to 8.6%, but the change may not be accurate, reports the study’s lead author Dr. Lara Akinbami, a medical officer at the government’s National Center for Health Statistics.

Researchers believe that many factors contribute to attacks, such as air pollution, respiratory infections in infancy, obesity, premature birth, and tobacco smoke. Because some of these factors change, understanding trends is complicated.

An example would be that air quality in the US has improved in general compared to earlier decades. But kids who attend schools that have been built near congested highways may still be negatively impacted by pollution. In the same vein, smoking rates in the US have declined, but climate warming may prolong the growing season and allow for more exposure to pollen.

Dr. Jeffrey Biehler, chair of pediatrics at Nicklaus Children’s Hospital in Miami, notes that the presence of asthma at higher rates in poor children is probably due to environmental factors such as tobacco smoke, smog, cockroaches, mold, mildew, dust mites, and pet dander. Additionally, the stress associated with low-income levels contributes to the risk of developing asthma.